Let it go. I have been through this, actually a recent break up. He was the rebound. We slept together the second night. He is 25. I am 29.

At 3 months, I wanted him to back off and slow down. I wanted his space, but he kept at me, didn't want to lose our magic. I wasn't emotionally ready to take on a relationship before working through the baggage of my previously jacked up relationship. Three years and a wretched break up later, I now realize, it wouldn't have gotten this far and nasty, if he would have just let me go when I was trying to find and heal myself.
If you care for her, give her this space, be her friend....don't be selfish. She needs to find herself and love herself before she can love you.